Andrei Garcia

Effective Cleaning of Hexagonal Boron Nitride for Graphene Devices

Hexagonal boron nitride (h-BN) films have attracted considerable interest as substrates for graphene. ( Dean, C. R. et al. Nat. Nanotechnol. 2010 , 5 , 722 - 6 ; Wang, H. et al. Electron Device Lett. 2011 , 32 , 1209 - 1211 ; Sanchez-Yamagishi, J. et al. Phys. Rev. Lett. 2012 , 108 , 1 - 5 .) We study the presence of organic contaminants introduced by standard lithography and substrate transfer processing on h-BN films exfoliated on silicon oxide substrates. Exposure to photoresist processing adds a large broad luminescence peak to the Raman spectrum of the h-BN flake. This signal persists through typical furnace annealing recipes (Ar/H(2)). A recipe that successfully removes organic contaminants and results in clean h-BN flakes involves treatment in Ar/O(2) at 500 °C.

Tunneling spectroscopy of graphene-boron-nitride heterostructures

We report on the fabrication and measurement of a graphene tunnel junction using hexagonal-boron nitride as a tunnel barrier between graphene and a metal gate. The tunneling behavior into graphene is altered by the interactions with phonons and the presence of disorder. We extract prop- erties of graphene and observe multiple phonon-enhanced tunneling thresholds. Finally, differences in the measured properties of two devices are used to shed light on mutually-contrasting previous results of scanning tunneling microscopy in graphene.

Spatially Resolved Study of Backscattering in the Quantum Spin Hall State

The discovery of the Quantum Spin Hall state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the Quantum Spin Hall state confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using Scanning Gate Microscopy to perturb the QSH edge states on a sub-micron scale, we identify well-localized scattering sites which likely limit the expected non-dissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed as expected for an ideal QSH system and are found to be robust against weak induced potential fluctuations.

Coaxial tip piezoresistive scanning probes for high-resolution electrical imaging

We have designed and batch fabricated silicon cantilever scanning probes integrating, for the first time, a coaxial tip to produce highly localized electric fields and a piezoresistor to measure cantilever deflection. These probes will improve the lateral resolution of scanning gate microscopy enabling the study of electron organization in semiconductor nanostructures. The full-width at half-maximum of the perturbation produced by our coaxial tip is ~3x smaller than that of conventional tips. At 300 K, the vertical displacement resolution of a 405 ¿m long probe is 2.4 nm in a 1 kHz bandwidth. With the ability to image topography and apply local electric fields, our probe has broad applications including electromechanical studies of cells and dopant profiling in semiconductors.

Self-sensing cantilevers with integrated conductive coaxial tips for high-resolution electrical scanning probe metrology

The lateral resolution of many electrical scanning probe techniques is limited by the spatial extent of the electrostatic potential profiles produced by their probes. Conventional unshielded conductive atomic force microscopy probes produce broad potential profiles. Shielded probes could offer higher resolution and easier data interpretation in the study of nanostructures. Electrical scanning probe techniques require a method of locating structures of interest, often by mapping surface topography. As the samples studied with these techniques are often photosensitive, the typical laser measurement of cantilever deflection can excite the sample, causing undesirable changes electrical properties. In this work, we present the design, fabrication, and characterization of probes that integrate coaxial tips for spatially sharp potential profiles with piezoresistors for self-contained, electrical displacement sensing. With the apex 100 nm above the sample surface, the electrostatic potential profile produced by o...

Fabrication of samples for scanning probe experiments on quantum spin Hall effect in HgTe quantum wells

We present a fabrication process for devices on HgTe quantum wells through which the quantum spin Hall regime can be reached without the use of a top-gate electrode. We demonstrate that a nominally undoped HgTe quantum well can be tuned from p-type to n-type, crossing through the quantum spin Hall regime, using only a back-gate hundreds of microns away. Such structures will enable scanning probe investigations of the quantum spin Hall effect that would not be possible in the presence of a gate electrode on the surface of the wafer. All processes are kept below 80 °C to avoid degradation of the heat-sensitive HgTe quantum wells.